SELECT * FROM nls_database_parameters WHERE PARAMETER = 'NLS_CHARACTERSET';
SELECT * FROM v$nls_parameters WHERE PARAMETER = 'NLS_CHARACTERSET';
-- NLS_CHARACTERSET for VARCHAR2, NLS_NCHAR_CHARACTERSET for NVARCHAR2
SELECT * FROM database_properties WHERE PROPERTY_NAME
    IN ('NLS_CHARACTERSET', 'NLS_NCHAR_CHARACTERSET');